The production of single crystal silicon wafers customarily involves growing a single crystal ingot, slicing the ingot into wafers, and then lapping, etching and polishing the wafers. Based upon the required specifications of a circuit device manufacturer, the silicon wafers may also be subjected to thermal processing, such as, but not limited to, oxygen donor annihilation annealing, thermal processing to control oxygen precipitation, low temperature chemical vapor deposition (CVD) oxidation, epitaxial deposition, and polysilicon deposition.
In the course of such thermal processing, a silicon wafer will typically be exposed to a temperature of at least about 300.degree. C. for a duration of at least about one second. Under these conditions, (contaminant) metals that may be present on the surface of the wafer, such as but not limited to nickel, copper, iron, chromium, calcium, titanium, cobalt, manganese, zinc and vanadium, can be driven into the silicon crystal material, where they can degrade bulk silicon minority carrier recombination lifetime. Ideally, the silicon wafer should be metal-free when subjected to thermal processing.
In many applications it is also preferred that silicon wafers to be subjected to thermal processing be passivated by a hydrophilic silicon oxide layer. Unfortunately, a number of limitations associated with conventional processes for growing hydrophilic surface layers of silicon oxide have made it impractical to grow such a silicon oxide layer at a surface concentration of less than 1.times.10.sup.11 atoms/cm.sup.2 of contaminant metals (minority carrier recombination lifetime killers). As a result, silicon wafers have been routinely stripped of their surface oxide layers prior to thermal processing. Unfortunately, stripping such oxide layers prior to thermal processing is not without its disadvantages, as silicon wafers that have a hydrophobic surface layer can be prone to localized (metal) contamination.
A proposal to solve this problem, described in the Pirooz et al. U.S. Pat. No. 5,516,730, is to initially immerse the wafer in a conventional pre-clean SC-1 aqueous cleaning solution containing H.sub.2 O.sub.2 and NH.sub.4 OH, in order to remove organic contaminants and particulates, and to form soluble complexes of contaminant metals such as iron, copper, gold, nickel, cobalt, zinc and calcium. To remove the complexed metals, the surface of the pre-cleaned silicon wafer is subjected to the flow of an aqueous solution hydrofluoric (HF) acid (which contains hydrochloric (HCl) acid to enhance metal removal), followed by rinsing the HF and HCl acid-treated wafer in deionized water. In order to grow a hydrophilic oxide layer on its surface, the rinsed wafer is then contacted with ozonated water. The ozonated water-treated wafer upon which the oxide layer has been grown is then heated to a temperature of at least about 300.degree. C. for a duration of at least about one second.
Now although the patentee states that the concentration of contaminant metals on the surface of the oxide-grown wafer at the beginning of the anneal/heating step is less than 1.times.10.sup.9 atoms/cm.sup.2, his use of ozonated water creates several problems that are antithetical to the objective of forming a hydrophilic oxide on the metal free surface of a silicon wafer. First of all, during the production of ozone in present day commercially available ozone generation equipment, contaminant metals may be leached from the hardware into the ozonated water. Thus, irrespective of how well cleaned the surface of the silicon wafer is following the post HF and HCl acid-treatment (deionized water) rinse, the silicon wafer surface may be exposed to minority carrier recombination lifetime killer contaminants in the ozonated water.
Secondly, ozone concentration in the produced solution is extremely difficult to control. As a result, the degree to which the ozone (which tends to be an extremely aggressive solution with respect to the surface of the silicon wafer) may modify the surface of the wafer can vary. Consequently, it may be expected that the silicon wafer surface will have a texture that may readily trap contaminant metals that are likely to be present in the ozonated water. When the oxide layer begins to grow, these minority carrier recombination lifetime killer metals become fixed or trapped on the wafer surface, and are eventually driven into the bulk silicon material during subsequent thermal processing.